The invention relates to postage meters and in particular to electronic postage meters having microcomputer control of printing and accounting functions.
Devices of this type are generally known, and are discussed for example in U.S. Pat. No. 3,978,457. This patent discloses a system for a postage meter which includes a keyboard for the manual introduction of data corresponding to the postage to be printed in a Random Access Memory for real time operation. Data is stored in a nonvolatile memory upon power down and read into the Random Access Memory upon power up.
U.S. Pat. No. 4,481,604 describes an electronic postage meter having a redundant memory system in which for each postal printing operation identical data is stored, respectively, in two separate but identical CMOS battery-backed nonvolatile memories.
In these known devices, there have been found to be times when essential data has not properly been stored in the nonvolatile memory of the meter. It has been found that one reason might be the improper selection of access to a particular device.
In known electronic postage meters, the microprocessor high order address bits or combination thereof are utilized in a standard decoder for selecting or enabling a particular memory or peripheral device to be accessed in accordance with the microcomputer instructions. While this normally works well, in many cases of improper operation of the microcomputer or failure of one of the address lines of the bus, an improper bit may be decoded and the select logic gate which then enables the wrong device may cause wrong data to be read from memory or in the worst case cause data to be written into an unknown memory or peripheral with no indication of any malfunction. When this happens there is a strong possibility of service personnel's not being able to recover essential information from the nonvolatile memory in the postage meter when the postage meter fails.